1. Field of the Invention
The present invention relates to a machine tool for machining a workpiece by moving a tool spindle in three directions mutually perpendicular to each other with respect to the workpiece, and more particularly, to a machine tool in the form of that a feed movement of a tool spindle in at least one direction is driven by a linear motor.
2. Description of the Related Arts
In this kind of machine tool disclosed in Japanese Patent Application Laid-Open (JP-A) No. 8-318445, a box frame 481 is arranged on a base 480 to guide a gantry 483 at upper and lower end portions thereof within a rectangular window 482 opened in the box frame 481, as shown in FIGS. 1 and 2. The gantry 483 is fed in an X-axis direction by a pair of linear motors 484 and 485 each disposed between the upper and lower end portions of the gantry 483 and opposite surfaces of the frame 481 thereto.
Further, the gantry 483 has a rectangular window 486 in which a saddle 487 is guided within the rectangular window 486 by a pair of right and left linear guide mechanisms (not shown) to be movable and fed in a Y-axis direction by a pair of right and left linear motors 488. A ram 490 is guided on the saddle 487 by a linear guide mechanism (not shown) to be movable forward and rearward in a Z-axis direction by a liner motor (not shown) The ram 490 also rotatably supports a tool spindle 490a rotated by a drive motor (not shown) around the Z-axis.
Each of the linear guide mechanisms for guiding the gantry 483 at left and right sides in the X-axis direction consists of a lower linear guide mechanism 491 including a guide rail and a bearing block running thereon, and an upper linear guide mechanism 492. The lower linear guide mechanism 491 is disposed between a lower surface of the gantry 483 and a lower beam member 481c of the box frame 481, and the upper linear guide mechanism 492 is disposed between a front surface of a cross beam 481a of the frame 481 and a back surface of an upper portion of the gantry 483 opposite thereto.
In the conventional machine tool driven by the linear motor mentioned above, since the gantry 483 is arranged so as to be movable within the rectangular window 482 opened in the frame 481, a stroke of the gantry 483 in the Y-axis direction is limited between right and left column members 481b of the frame 481.
Further, the cross beam 481a of the frame 481 is arranged to cover an upper end surface of the gantry 483, so that a height of the frame 481 increases, thereby being enlarged in the volume of the machine tool.
Furthermore, since the lower linear guide mechanism 491 for guiding the lower end portion of the gantry 483 in the X-axis direction is mounted to a lower surface of a base plate 483a of the gantry 483, in the case of inspection, maintenance and replacement of the linear guide mechanism and the linear motor 488 for moving in the Y-axis direction the saddle 487 mounted on the right and left end surfaces of the rectangular window 486 in the gantry 483, it is required to separately remove all the elements provided on the base plate 483c of the gantry 483. Therefore, this may causes a troublesome in reassemble work of the linear guide mechanism and the linear motor the after the inspection, maintenance and replacement operation thereof.
Next, a machine tool disclosed in Japanese Patent Application Laid-Open(JP-A) No. 8-318445 will be explained in further detail with reference to schematic drawings of FIGS. 3 and 4 with respect to the prior art.
In FIG. 3, the gantry 483 is provided in a frame (not shown) through the pair of upper and lower linear guide mechanisms 491 and 492 for being slidably driven along the X-axis. The linear guide mechanisms 491 and 492 for the X-axis drive are arranged in parallel in the Y-axis direction in which the tool spindle 490a is rotatably supported around the Z-axis therebetween. The lower linear guide mechanism 491 is composed of a straight rail 101 extending along the X-axis and right and left bearing blocks 103 and 104 slidably engaged therewith. The straight rail 101 is fixed to the frame, and the bearing blocks 103 and 104 are fixed to the gantry 483.
Similarly to the lower linear guide mechanism 491, the upper linear guide mechanism 492 is composed of a straight rail 102 fixed to the frame and right and left bearing blocks 105 and 106 which are slidably engaged therewith and fixed to the gantry 483. X-axis guide means for slidably guiding the gantry 483 along the X-axis consists of the pair of upper and lower X-axis linear guide mechanisms 491 and 492 as described above. Further, in the X-axis guide means, four bearing blocks 103, 104, 105 and 106 fixed to the gantry 483 are arranged on the same plane perpendicular to the Z-axis.
Further, the saddle 487 is provided on the gantry 483 through Y-axis linear guide mechanisms 422 and 424 so as to be slidable along the Y-axis. The Y-axis linear guide mechanisms 422 and 424 are disposed in parallel to the X-axis direction in which the tool spindle 490a is rotatably supported therebetween. The right Y-axis linear guide mechanism 424 is composed of a straight rail 110 extending along the Y-axis and lower and upper bearing blocks 112 and 114 slidably engaged therewith. The straight rail 110 is fixed to the gantry 483, and the bearing blocks 112 and 114 are fixed to the saddle 487.
Similarly to the right Y-axis linear guide mechanism 424, the left Y-axis linear guide mechanism 422 is composed of a straight rail 111 fixed to the gantry 483 and lower and upper bearing blocks 113 and 115 which are slidably engaged with the straight rail 111 and fixed to the saddle 487. The Y-axis guide means for slidably guiding the saddle 487 in the Y-axis direction consists of the pairs of right and left Y-axis linear guide mechanisms 422 and 424 mentioned above. Further, in the Y-axis guide means, four bearing blocks 112, 113, 114 and 115 fixed to the saddle 487 are disposed on the same plane perpendicular to the Z-axis.
Further, in FIG. 4, the ram 490 is provided in the saddle 487 through Y-axis linear guide mechanisms 432 and 434 slidable along the Z-axis. Z-axis linear guide mechanisms 432 and 434 are disposed in parallel in the X-axis direction downward of the tool spindle 490a. The right Z-axis linear guide mechanism 434 is composed of a straight rail 125 extending along the Z-axis and back-and-forth bearing blocks 121 and 123 slidably engaged therewith. The straight rail 125 is fixed to the ram 490, and the bearing blocks 121 and 123 are fixed to the saddle 487.
Similarly to the right Z-axis guide mechanism 434, the left X-axis linear guide mechanism 432 is composed of a straight rail 126 fixed to the ram 490 and back-and-forth bearing blocks 122 and 124 which are slidably engaged therewith and fixed to the saddle 487. Z-axis guide means for slidably guiding the ram 490 along the Z-axis consists of the pair of left and right Z-axis linear guide mechanisms 432 and 434, as described above. Further in the Z-axis guide means, four bearing blocks 121, 122, 123 and 124 fixed to the ram 490 are disposed on the same plane perpendicular to the Y-axis.
In this machine tool, the four bearing blocks 103, 104, 105 and 106 of the X-axis guide means disposed on the same plane perpendicular to the Z-axis to support the gantry 483 are fixed to a front surface thereof. Similarly, the four bearing blocks 112, 113, 114 and 115 of the Y-axis guide means for supporting the saddle 487 are fixed to a front surface thereof. In such a construction, cutting forces acting on the tool spindle 490a are concentrically applied in a cutting operation of the workpiece to each of the front surfaces of the gantry 483 fixing the bearing blocks 103, 104, 105 and 106 and of the saddle 487 fixing the bearing blocks 112, 113, 114 and 115, respectively.
Each of the front surfaces of the gantry 483 and the saddle 487, thus, is required to be high in rigidity to react with the cutting force acting on the tool spindle 490a. In the conventional machine tool, the rigidity of the front surfaces is enhanced by increasing a thickness of each front surfaces of the gantry 483 and the saddle 487. However, since the thickness increase of the front surfaces in the gantry 483 and the saddle 487 does not effectively act on the other portion thereof with respect to the cutting force, the resultant weight is excessively increased, thereby preventing the weight reduction of the gantry 483 and the saddle 487.
Further, the ram 490 is supported by the four bearing blocks 121, 122, 123 and 124 of the Z-axis guide means disposed on the same plane perpendicular to the Y-axis, this causes a similar problem to that in the gantry 483 and the saddle 487. Generally, it is contrary problems to each other to require to reduce the weight of the slide members in the gantry 483, the saddle 487 and the ram 490 and to enhance the rigidity thereof. However, it is particularly important to simultaneously satisfy the problems as mentioned above, in the machine tool driven by the linear motor in which the slide member is positioned by the linear motor at a high speed.